Here's why:
* Slang is a subset of a language: It exists within a particular language, like English, Spanish, or French. It's not a separate, independent language.
* Slang is informal: It's used in casual settings and often changes rapidly.
* Slang varies by region and group: What's slang in one part of the country or among a certain group may not be slang in another.
Think of it this way: Slang is like a dialect within a language. It's a specific way of speaking that uses unique vocabulary and expressions.
For example, "chill" and "lit" are examples of slang words in English. They are not found in formal dictionaries or used in formal settings, but they are widely understood among certain groups of English speakers.